1. Field of the Invention
The present invention relates to a management technique of a data hold time required by a file memory, and in particular, to a management technique applied to a high recording density file memory card requiring an inexpensive, downsized built-in battery.
2. Description of the Related Art
A memory card used in an external device such as a portable cellular phone or a personal computer must comprise a (removable) function capable of holding data even if the card is removed from the external device and not in use.
However, with respect to a file memory incorporated on a memory card, the downsizing of a memory cell advances to achieve low bit cost and high recording density. Thus, there occurs a problem that it becomes difficult to maintain a constant data hold time.
For example, by way of example of a NAND-type flash memory used as a file memory, if thinning a gate insulating film between a channel and a floating gate electrode and an inter-poly-insulating film between the floating electrode and the control gate electrode advances, a charge in the floating gate electrode is likely to leak. As a result, a data hold time is reduced (reference should be made to H. Wo et al. IEEE Trans. ED52, (5) 955 (2005), for example).
The thinning of the gate insulating film and the inter-poly-insulating film is indispensable from the viewpoint of strengthening capacitance coupling in memory cells and precluding interference between the memory cells due to downsizing. Thus, the problem with this reduced data hold time must be solved from other points of view.
First, from the viewpoint of a material, there is known a technique of using an inter-poly-insulating film made of a high dielectric material (reference should be made to W. H. Lee et al. Tech. Dig. VLSI symp; 117 ('97)).
However, at the present stage, there has not been found a material having two properties, good affinity with a silicon process and infrequent occurrence of leakage.
In addition, the problem with reduced data hold time cannot be sufficiently solved merely by changing a material. This is because the thinning of a tunnel insulating film and the inter-poly-insulating film results in more dispersion in memory. In this case, if the insulating film of the memory cell whose thickness deviates to be thin fail to meet a condition for a data hold time (for example, one year or more), it causes the lowered yield of manufacturing.
Second, from the viewpoint of a system, there is known a technique of refreshing memory cell data by using a battery (reference should be made to Jap. Pat. Appln. KOKAI publication No. 2004-280971).
In addition, a card incorporating a battery is also known (for example, reference should be made to Japanese Patent Application Laid-open No. 2000-358090 and U.S. Pat. No. 6,707,748). In order to actually incorporate batteries in these cards, power required for clock generation or timer operation and the like must be reduced, and battery capacitance must be reduced.
Third, as a nonvolatile memory incorporated in a memory card, there is known a technique of using a novel memory such as a ferromagnetic memory, an MRAM (magnetic random access memory), an OUM (ovonics unified memory) as a phase change RAM, instead of the NAND-type flash memory.
However, the ferromagnetic memory and MRAM are not suitable for a file memory because a cost per bit is high as compared with that of the flash memory. In addition, the OUM is high in power consumption during write/erase operation, and a technique of reducing an off-leak without increasing a bit cost still remains unsolved.
Further, with respect to these novel memories as well, there occurs a problem that data hold characteristics are impaired due to thermal disturbance.
On the other hand, there has been reported a semiconductor integrated circuit having incorporated therein an electronic timer which does not require a battery, in particular, a semiconductor integrated circuit composed of an aging device whose output changes with time (reference should be made to Jap. Pat. Appln. KOKAI publication No. 2004-172404, for example).